Ceramic block filters offer several advantages over lumped component filters. The blocks are relatively easy to manufacture, rugged, and relatively compact. In the basic ceramic block filter design, the resonators are formed by typically cylindrical passages, called through-holes, extending through the block from the long narrow side to the opposite long narrow side. The block is substantially plated with a conductive material (i.e. metallized) on all but one of its six (outer) sides and on the inside walls formed by the resonator through-holes.
One of the two opposing sides containing through-hole openings is not fully metallized, but instead bears a metallization pattern designed to couple input and output signals through the series of resonators. This patterned side is conventionally labeled the top of the block. In some designs, the pattern may extend to sides of the block, where input/output electrodes are formed.
The reactive coupling between adjacent resonators is dictated, at least to some extent, by the physical dimensions of each resonator, by the orientation of each resonator with respect to the other resonators, and by aspects of the top surface metallization pattern. Interactions of the electromagnetic fields within and around the block are complex and difficult to predict.
These filters may also be equipped with an external metallic shield attached to and positioned across the open-circuited end of the block in order to cancel parasitic coupling between non-adjacent resonators and to achieve acceptable stopbands.
Although such RF signal filters have received widespread commercial acceptance since the 1980s, efforts at improvement on this basic design continued.
In the interest of allowing wireless communication providers to provide additional service, governments worldwide have allocated new higher RF frequencies for commercial use. To better exploit these newly allocated frequencies, standard setting organizations have adopted bandwidth specifications with compressed transmit and receive bands as well as individual channels. These trends are pushing the limits of duplex filter technology to provide sufficient frequency selectivity, increased band isolation, decreased insertion loss, decreased band interference, and reduced cross-talk.
Coupled with the higher frequencies and crowded channels are the customer trends towards the use of the same printed circuit board and filter across the different operating frequencies of different frequency platforms and the consumer market trends towards ever smaller wireless communication devices and longer battery life. Combined, these trends place difficult constraints on the design of wireless components such as filters. Filter designers may not simply add more space-taking resonators (i.e., increase the size of the filter) or allow greater insertion loss in order to provide improved signal rejection.